In a blender or food processor, process the bread to make fine crumbs.
In a large nonstick frying pan, heat 1 1/2 teaspoon olive oil over medium heat. Add the sliced garlic and sauté until lightly golden, about 1 minute. Stir in the bread crumbs and cook until lightly browned and crunchy, 3-4 minutes.
Transfer to a bowl and stir in the walnuts, parsley, and 1/2 teaspoon the salt: set aside.
Add the remaining 2 tablespoons oil to the pan and heat over medium heat.
Add the yellow squash, zucchini, and carrot and sauté until the vegetables are tender-crisp, about 5 minutes. Transfer to a plate and keep warm.
Add the bell peppers to the pan and sauté until they begin to soften, about 2 minutes. Stir in the remaining 1/2 teaspoon salt and the pepper. Return the squash mixture to the pan and toss to mix. Set aside and keep warm.
Fill a large pot three-fourths full with water and bring to a boil. Add the spaghetti and cook until al dente, 10-12 minutes, or according to package directions. Drain the pasta thoroughly.
In a warmed shallow serving bowl, combine the spaghetti, vegetables, and bread crumb mixture. Toss gently to mix.
Serve immediately. This dish does not reheat well.